If you're having a hard time replying to text messages this morning and are being told you're trying to reply to an invalid number ... you're not alone.
We're still diving into this a bit, but it looks like incoming text messages (and this appears to be localized on AT&T) are being appended with a +11 country code. Problem is there is no 11 country code, so the reply fails.
Update: According to AT&T's social team, the issue has been fixed but there's a step or two on your device that needs to happen for everything to run smoothly again. You'll need to delete the SMS thread that contains messages with +11 in them, and the next message you receive should work as intended. We've tested this method here and confirmed that messaging capabilities have been restored without issue.